Re: How much do the hint bits help?

On 22.12.2010 15:59, Simon Riggs wrote:
> On Wed, 2010-12-22 at 15:30 +0200, Heikki Linnakangas wrote:
>> My gut feeling is that a reasonable compromise is to set hint bits like
>> we do today, but don't mark the page as dirty when only hint bits are
>> set. That way you get the benefit of hint bits for tuples that are
>> frequently accessed and stay in buffer cache. But you don't spend any
>> extra I/O to set them. I'd really like to see a worst-case scenario
>> benchmark of a patch that does that.
>
> That sounds great, but still prevents block checksums and that is a very
> valuable feature for robustness.

It does? The problem with block checksums is that if you modify a page
and don't have a corresponding WAL record for it, like a hint bit
update, you can have a torn page so that the checksum doesn't match.
Refraining from dirtying the page when a hint bit is updated avoids the
problem. With that change, we only ever write pages to disk that have a
WAL record associated with it, with full-page images as necessary to
avoid torn pages.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com